Illegal immigration imposes enormous costs -- monetary as well as crime-related -- on American society. As regards criminal activity, Manhattan Institute scholar Heather MacDonald describes one small slice of a much larger problem:

In Los Angeles, 95 percent of all outstanding warrants for homicide target illegal aliens, as do approximately two-thirds of all fugitive felony warrants.

More than 60 percent of the Hispanic gangs in Southern California—whose membership is in the tens of thousands—is illegal. These gangs involved withdrug-distribution schemes, extortion, drive-by assassinations, assaults, and robberies.

In a 2006 study, Deborah Schurman-Kauflin of the Violent Crimes Institute in Atlanta estimated, conservatively, that from January 1999 through April 2006 approximately 240,000 illegal aliens had committed about 960,000 sex offenses in the United States.

The fiscal costs of illegal immigration are also very high. According to the Center for Immigration Studies, in 2002 illegal-alien households imposed, in aggregate, costs exceeding $26 billion on the federal government while they paid $16 billion in federal taxes -- thereby creating a net fiscal deficit of $10.4 billion per year at the federal level, or $2,700 per household. Among the largest components of this deficit were Medicaid ($2.5 billion); medical treatment for the uninsured ($2.2 billion); food-assistance programs such as food stamps, WIC, and free school lunches ($1.9 billion); the federal prison and court systems ($1.6 billion); and federal aid to schools ($1.4 billion). A major reason why illegal aliens are, on balance, such a drain on the American Treasury is because approximately 60 percent of them lack a high-school degree.

The National Academy of Sciences has estimated that the average immigrant without a high-school degree will, over the course of his or her lifetime, impose a net cost -- above and beyond any taxes he or she pays -- of nearly $100,000 on U.S. taxpayers; this cost does not include the cost of educating the immigrant’s children. Based on that figure, the estimated 6 million legal immigrants lacking a high-school diploma and residing in the U.S. today, will cost taxpayers more than a half trillion dollars over their lifetimes.